alleryWhich of the following is **always** true for a spontaneous process in an isolated system?
A$\Delta H < 0$
B$\Delta G < 0$
C$\Delta U < 0$
D$\Delta S_\text{system} > 0$
Answer & Solution
Correct answer: D. $\Delta S_\text{system} > 0$
For an isolated system there is no exchange of heat or matter with surroundings, so $\Delta S_\text{surr} = 0$.
The second law then requires the *total* entropy change to be non-negative for a spontaneous process, which reduces to $\Delta S_\text{system} > 0$.
$\Delta G$ as a spontaneity criterion is meaningful only at constant $T$ and $p$ with the surroundings (not isolated).
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